The present invention is generally directed to a system and process for transferring nonwoven webs from a first conveyor to a second conveyor in a wet papermaking process. More particularly, the present invention is directed to a process for transferring a paper web at a low solids consistency from a first fabric to a second fabric shortly after the web has been formed. In one embodiment, the nonwoven web is formed in between a first forming fabric and a second forming fabric and is transferred solely to the second forming fabric as the fabrics diverge.
In making various paper products, typically an aqueous suspension containing pulp fibers is first formed. The aqueous suspension is then spread out over a forming surface in order to form a paper web. The forming surface generally includes a series of endless conveyors which are formed from a porous fabric that can be made from metal, plastic, or any other suitable material. The forming fabrics are designed to facilitate formation of the nonwoven web, to transport the nonwoven web, and to remove excess liquid from the web as it travels downstream.
In one embodiment, especially when forming low basis weight paper products such as tissues, the nonwoven web is formed in between a pair of forming fabrics. More specifically, in these systems, an aqueous suspension of fibers is injected on to one or in between a pair of moving fabrics as the fabrics are being wrapped around a roll, which is generally referred to as a forming roll. The forming roll assists in draining liquids from the web. These types of systems are typically referred to as xe2x80x9croll formersxe2x80x9d. Examples of roll formers include twin-wire systems and crescent-former systems.
From the forming fabrics, the nonwoven web is usually transported through a press section and then through one or more driers. Depending upon the paper product being formed, the nonwoven web can then be subjected to various post formation processes as desired.
One problem that is typically encountered during the formation of paper products is that the forming and transferring fabrics have a tendency to become fouled and clogged by bonding materials, additives contained within the fiber suspension, and especially, by paper fibers, which are not transferred to the next process such as press or drying sections and are referred to as xe2x80x9cfiber carry backxe2x80x9d. Too much debris and fiber carry back on the fabric can create fiber waste and also can adversely affect sheet formation. The problems with fiber carry back become especially severe when the sheet being formed has a relatively low basis weight such as when making tissue paper, when short fibers at a low consistency are being used to form the paper sheet, at higher machines speeds, and when excessive amounts of fiber carry back begin to accumulate on the fabrics.
Fiber carry back typically occurs when a newly formed nonwoven web is transferred off of a forming fabric. As such, a need currently exists for a system and process for transferring a nonwoven web between fabrics that reduces fiber carry back. In particular, a need currently exists for an improved web transfer system that can efficiently transfer a newly formed nonwoven web at a low consistency or solids content from a first fabric to a second fabric without creating an unacceptable accumulation of fiber carry back. A need further exists for a nonwoven web transfer system for use in wet papermaking roll formers.
In the past, various systems and processes have been proposed that assist or facilitate the transfer of a nonwoven web from a first fabric to a second fabric. For instance, U.S. Pat. No. 5,830,321 to Lindsay. et al., which is incorporated herein by reference, discloses a method for improving the rush transfer of a nonwoven web between two separate fabrics that are moving at different speeds. Various features, aspects and advantages of the present invention, however, remain absent from the prior art as will be made apparent from the following description.
Accordingly, it is an object of the present invention to provide a process and system for transferring a nonwoven web between fabrics.
Another object of the present invention is to provide a system and process for transferring a nonwoven web to a moving fabric in a wet papermaking process while minimizing fiber carry back.
Still another object of the present invention is to provide a system and process for transferring a nonwoven web to a fabric using a vacuum shoe and an air knife.
These and other objects of the present invention are achieved by providing a nonwoven web transfer system and method for use in a wet papermaking process. The system includes a first fabric and a second fabric that are configured to receive a nonwoven web therebetween formed from an aqueous suspension of fibers. The first and second fabrics diverge whereby the nonwoven web is transferred solely to the first fabric.
In order to facilitate transfer to the first fabric, the system includes a vacuum shoe positioned against the first fabric at a location where the first fabric diverges from the second fabric. The vacuum shoe defines a vacuum slot configured to apply a suction force to the nonwoven web as the web is transferred to the first fabric.
A transfer shoe is positioned against the second fabric at a location generally opposite the vacuum shoe. The transfer shoe defines an air knife configured to deliver a pressurized gas against the nonwoven web as the web is being drawn towards the vacuum shoe. The air knife includes an air nozzle for emitting a pressurized gas.
Preferably, the second fabric is wrapped around the transfer shoe such that the second fabric forms an arriving angle and a departing angle with the transfer shoe. Further, the second fabric should be wrapped around the transfer shoe under sufficient tension such that the pressurized gas is forced through the fabric, as opposed to elevating the fabric off of the transfer shoe and allowing the air to flow around the fabric. In this regard, the tension place upon the fabric multiplied by the tangent of the departing angle should be greater than or equal to one half of the pressure of the gas being emitted by the air knife multiplied by the width of the gas channel.
The system and method of the present invention are particularly well suited for use in papermaking processes that produce low basis weight products, such as tissues. In this regard, the web transfer system of the present invention is well suited for use in roll forming systems. In these systems, an aqueous suspension of fibers is injected in between a first forming fabric and a second forming fabric as the fabrics are being wrapped around a forming roll. From the forming roll, the fabrics diverge and the base web is transferred solely to one of the fabrics.
As described above, the system of the present invention includes a vacuum shoe generally in alignment with a transfer shoe. In one embodiment, the leading edge of the gas channel is aligned with the leading edge of the vacuum slot. Further, for most applications, the vacuum slot can have a width greater than the width of the gas channel.
The gas that is emitted by the transfer shoe can be any suitable gas, such as air. The gas can be emitted at a pressure of at least 1 psi, and particularly from about 3 psi to about 15 psi.
The vacuum shoe and the transfer shoe generally include a convex surface which contacts the forming or transfer fabrics. In one embodiment, the vacuum shoe and the transfer shoe are stationary. The vacuum shoe and the transfer shoe can be made from various materials, including ceramics and plastics.
Other objects, features and aspects of the present invention are discussed in greater details below.